GROUND DEMOLITION WOOD IS NOT WORTHY OF BEING CALLED MULCH
Francis R. Gouin
Department of Horticulture
University of Maryland, College Park

Today's crushing and grinding equipment combined with magnetic belts now make it possible to convert used pallets, discarded wooden furniture, tree trunks and condemned wooden buildings into woodchips. Whole tree chippers are also being used to convert lumbering waste as well as dead and drying trees, many infested with diseases and insects, into woodchips. The resulting chips are sometimes marketed as mulch immediately after processing, or stacked into large piles for ageing or colored by mixing with either pine or hardwoods bark mulch for several weeks. The fines resulting from screening are in some instances sold as soil amendments to unsuspecting buyers.

This latest effort in recycling is attracting many new entrepeneurs and crating numerous horticultural problems. Every "Tom, Dick and Harry" in waste wood recovery is looking at the mulch market to increase their profits. Since they already collect tipping fees for allowing the wood to be dumped on their property and to pay for grinding, selling the ground wood as mulch is clear profit. To these individuals, mulch is wood waste that is spread on the ground. They are not aware that quality bark mulch consists mostly of lignatious materials that decompose slowly. Woodchips from demolition, trees, branches and tree trunks consist mostly of cellulose and starches that must undergo decomposition through composting before they can be used. To the unsuspecting buyer, it may look like great stuff at a great price. Even the experienced landscape contractors are buying it because it's cheap.

The end result from this new recycling effort is a glut of nitrogen (N) robbing, disease and insect-infested wood waste, sometimes loaded with alcohol and acetic acid. When applied as a mulch around young trees, shrubs, herbaceous perennials and annuals, it will rob N from plants, causing them to become stunted with yellow green bottom leaves, often dropping prematurely. If the material has been stored in large piles it is not uncommon for the wood waste to accumulate sufficient amounts of alcohol and/or acetic acid to kill plants.

When ground demolition wood and woodchips are used as mulch it controls weeds by suffocation and by starving them from N. However, it will also starve the desired landscape plants for N at the same time. Newly planted plants and shallow rooted species will be those most vulnerable to injury.

The problem with using ground demolition wood and woodchips as mulch is due to an imbalance in the carbon/nitrogen (C/N) of the materials. For demolition wood the C/N/ ratio is between 700/1 and 800/1 while the C/N for woodchips from a branch, tree trunks and whole-tree chipper is between 200/1 and 300/1. Most of the carbon in both of these sources is in the form of cellulose and starches. These carbon sources are very different from those of pine bark or well-composted hardwood barks which consists mostly of lignins. Although lignins are a source of carbon, they are resistant to decomposition. Their rate of decomposition is so slow that it does not compete with plant roots for N. Therefore, to utilize ground demolition wood and woodchips as a mulch, their C/N ratio must be reduced to 60/1 through composting.

Because woodchips are a rich source of energy, in the form of carbon (C) fro microorganisms, using them as mulch without composting, will cause severe competition for available soil N between the microorganisms and the roots of the plants. Since the affinity for N by microorganisms is greater than that of plants, newly planted plants and shallow rooted species will be most severely affected. The plants generally respond by dropping their older leaves, stop growing and generally are more susceptible to diseases and insects. The lower leaves are the first to be affected because the N remaining in the plant is transferred from older leaves to the growing points. Although newly planted plants are more susceptible than established plants, established plants will also succumb to the same symptoms, but it will take longer to become evident.